With the rapid development of computer and communication technologies, the carrying of voice, data, image and other kinds of messages via public network has become a major business line that network service providers endeavor to develop. Network service providers generally rely on telecommunication service networks to provide all kinds of comprehensive and open network services involving voice, data, multimedia and so on. While the construction modes of telecommunication service networks are presently under change, an important objective of development is to separate application services from transmission technology, so that all application services can work on any type of transmission technology without restriction. In other words, the telecommunication service networks in the future will be service-oriented networks, where network services are separated from network transmission control, so as to achieve the goal that network application services are truly independent beyond the network transmission and can be offered flexibly and effectively. Therefore, in the future telecommunication service networks, users will be allowed to arrange and define their service characteristics by themselves without worrying about the types of network transmissions and the types of terminals. This feature will make services and applications more flexible. In other words, it is originally impossible for any existing information network, such as a telecommunication network, a computer network or a cable television network, to be the sole basic platform and to develop services with attributes different from the attributes of the network itself. However, with the development of IP technology in recent years, it is now possible to apply advanced IP technology to integrate telecommunication networks (wire-based and wireless), computer networks and cable television networks into a unified network, or is called the “three-in-one” network. This network may further merge with the mobile communication networks to the so-called “four-in-one” network, where a variety of IP-based businesses can communicate with one another on different networks through IP protocols.
Urged by the aforesaid network-based businesses and under the trend of network integration, future telecommunication service networks will adopt a distributed network structure, whose application functions, control functions and transmission functions are independent of one another, to effectively transmit voice, video and multimedia information within different networks. During network construction, the existing information networks will be able to change smoothly to the future telecommunication networks, with the advantage that existing network equipments and terminal equipments can be directly used to the future telecommunication networks. In addition, many of the existing IP-based protocols will still be used. However, how to traverse private networks is the most serious problem.
More particularly, a future telecommunication service network will be one integrated with voice, data, multimedia and mobile services, and can be divided into the following network layers:    (1) Access layer: The access layer is composed of various gateways, intelligent access terminal equipments and integrated access equipments. It uses a plurality of accessing approaches (including broadband accessing, mobile accessing, etc.) to make different kinds of users connect to the network and to convert information formats into those capable of being transmitted on the network.    (2) Transmission layer: The transmission layer refers to the carrying network in the telecommunication service network and serves to provide a public transmission platform for transmitting all kinds of information and media streams in the way mostly by adopting packet transmission while so far the main transmission network would be the broadband IP network.    (3) Control layer: In this layer, comprehensive control and processing functions such as invocation processing control, access protocol selection, interconnection and intercommunication are carried out while business logic is substantially executed. The control layer determines the businesses a user receives, and controls the processing of business flows by low-level network elements. The control layer is implemented mainly by a soft switch equipment.    (4) Business layer: This layer handles business logic, provides user-oriented integrated services, and performs business customization and business-related management functions, such as business verification and business billing.
The traversal problem in broadband access of a future telecommunication service network is no other than the broadband access problem of an edge access layer in the telecommunication service network. More particularly, the core carrying network and broadband access of a future telecommunication service network are built on existing IP networks, where access users must be addressed via IP addresses. However, the fact is, as the Internet speedily expands, IP addresses are seriously depleting. To solve this problem, a great number of corporate networks and local area networks deploy Network Address Translation (NAT) equipment at their network outlets. A NAT, an Internet standard defined in RFC 1631, is typically installed in a router located at the border of a private network and a public network for performing IP address translation on packets sent from a network terminal device in the private network, so that a plurality of network terminal devices in the private network are allowed to share a public IP address when connecting to the Internet. More specifically, when an IP data packet sent from the private network reaches the NAT equipment, the NAT equipment translates a private network IP address into a legal IP address in the public network. When a packet sent from outside the private network reaches the NAT equipment, the NAT equipment translates a public network address into a private network address by checking with data in a mapping table stored in the NAT equipment, and then forward the packet to an internal receiving point. For a general data packet, the NAT equipment usually only has to conduct IP address and port number translation. However, in applications of such standards as H.323, Session Initiation Protocol (SIP) and Media Gateway Control Protocol (MGCP), the real media connection data is stored and delivered in a data packet loading, which give rise to the following problem. Assuming a terminal A initiates a calling to a terminal B, and a soft switch relays the calling information of the terminal A to the terminal B, then according to protocols such as H.323 and SIP, the terminal B will acquire from the data packet loading a private network IP address of the terminal A and then attempts to establish a Real-time Transport Protocol (RTP) connection with the terminal A. Nonetheless, the IP address, which is private, cannot be used to establish a communication connection between the terminals A and B because private addresses are not recognizable in public networks. As a result, the NAT only allows an internal network terminal device to set up an outbound communication connection but does not allow a network terminal device outside the network to communicate with network terminal devices within the network. Therefore, although the NAT is capable of ensuring network security, it also blocks communication signals (such as VoIP phone signals) coming from external networks, so that the NAT protocol becomes a technical barrier for enterprise customers in using network communication services (such as VoIP phone services).
In view of this, network service and software providers (such as Microsoft and Cisco) are now in collaboration to develop a new Interactive Connectivity Establishment (ICE) protocol and intend to incorporate the ICE protocol into application programs (such as application programs of Microsoft) which, when employed in a network system by using the NAT protocol, allow network terminal devices within and outside the network to exchange NAT equipment data (such as NAT types and port differentials λ of a NAT equipment) in a controlled manner. However, the network protecting function of a NAT equipment still blocks network communication signals coming from external networks, preventing the network terminal devices from directly establishing connection channels therebetween. Consequently, peer-to-peer connection is unattainable and mutual data access cannot be achieved.
Therefore, if a direct peer-to-peer connection is established between network terminal devices of different private networks, the aforementioned problem with NAT equipments between different private networks must be solved. When two network terminal devices located respectively in two private networks try to set up a direct peer-to-peer connection, one of the network terminal devices has to know the endpoint mapping of the other network terminal device in the peer nodes. For example, the current network system where IPv4 addresses (i.e., addresses each having a 32-bit length and an address format of 210.130.1.1) are used, the endpoint mapping refers to mapping data of an IP address and a port number. Hence, the prerequisite for carrying out peer-to-peer connection between two network terminal devices is that two network terminal devices are able to acquire the endpoint mapping of each other. However, in a symmetric NAT equipment, endpoint mappings are not constant values, but are dynamically adjusted according to a preset port differential λ in the NAT equipment each time a packet is sent and received. Therefore, it is extremely difficult for a network terminal device of a private network to directly establish a peer-to-peer connection via a symmetric NAT equipment connected thereto with a network terminal device of another private network. To overcome this problem, attempts have been made to estimate, by predicting the range of, port numbers of a symmetric NAT equipment that are available for peer-to-peer connection. In practice, however, the symmetric NAT equipment dynamically allocates port numbers of packets sent and received so that, unless all the 65535 ports are completely detected via packets sent and received, it is impossible to accurately predict the range of port numbers available for the peer-to-peer connection. Therefore, inefficiency and insufficient accuracy are obviously two major drawbacks of the conventional approach of predicting the range of port numbers.
Consequently, it has been a major goal for many network service providers to design a network system wherein a network terminal device of a private network can speedily and accurately predict the range of port numbers of a symmetric NAT equipment in another private network, so that network terminal devices in the two private networks may promptly establish a connection channel for mutual communication or data access.